Series compensation

The corresponding frequency response is obtained by substituting i to for s (Section 7.8.4), viz  

The lead compensator contributes phase advance to the system and thus increases the overall system stability (Section 7.10.4). The degree of phase advance provided is a function of frequency. At the same time this type of compensator increases the overall system amplitude ratio, which has the effect of reducing the the stability of the system. However, the major contribution of phase advance occurs at those frequencies where the open-loop polar plot is adjacent to the (-1,0) point on the complex plane. The increase in amplitude ratio takes place at lower frequencies and, consequently, the effect of this is much less significant. As the ratio of r,/r2 is increased, the maximum phase advance supplied by the lead compensator also increases, i.e. the greater is the stabilising effect of the compensating element011. 

Curve a in Fig. 7.62 shows the open-loop polar plot for the heat exchanger system described in Example 7.6. with Kc = 1.8 and t,= 2.5 (see also Example 7.8 and Fig. 7.55). Clearly this indicates an unstable system (Section 7.10.5). If a lead compensator with r, = 1 min and r2 = 0.1 min (r,/r2 = 10) is inserted into the loop, as shown in Fig. 7.63, then the system becomes stable (curve b in Fig. 7.62) due to the additional phase lead supplied by the compensator. (Using these values of r, and r2, Kc can now be increased by almost a factor of ten before the system becomes unstable). 

Note that it is necessary to include an extra element r,/r2 in the loop in order to maintain the original open-loop steady-state gain. 

Polar plot of the open-loop transfer function for the heat exchanger control system described in Example 7.6 with Kc = 1.8 and t, 2.S (a) uncompensated system (b) compensated system with rt 1 min, r2 = 0.1 min 

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When the systems that are series compensated are eonneeted to a lightly loaded transformer or shunt reactor, under certain line conditions (Section 24.4). 

Rating of series capacitors 24/779 Advantages of series conrpensation 24/780 Analysis of a. systenr for series compensation 24/782... 

Figure 24,1 A simple transmission network with series compensation...

To neutralize atid reduce substantially the content of inductive reactance of the line. Refer to a simple transmission network with series compensation, show n in Figure 24.1,... 

A series compensated network w ill hav e its naltirtil frequency expiesscd by... 

These oscillations can be dampened with the use of filter circuits or by bypassing all or part of the series compensation dtiring a line disltirbtmce. Similar techniques arc adopictl while protecting... 

Since the line impedance, R + J (Xl - Xc), will reduce with a series compensation, the fault level of the system will rise. It should not matter if the fault level of the system is determined by the impedance of the source of supply, ignoring any other impedance of the circuit (Section 13.4.1 (5)). Moreover, such a situation is automatically averted through the protection of the series capacitors, as discussed below, by which the capacitors are bypassed during a line fault, the line restoring its original impedance, hence the original fault level. Nevertheless, when it is required to limit the system fault level, inductive coupling circuits may be provided to reduce the fault to the desired level. This is also discussed below ... 

In smaller cross-sectional area,s of the current-carrying conductors of the distribution network, i.e. for low-capacity networks where R/Xl is high, series compensation may be redundant. 

For higher cross-sectional areas, i.e. for high-eapacity networks where / /Xl is low, series compensation will be useful. 

Figure 24.6(a) Circuit diagram of a series compensated line... 

Figure 24.6(b) Phasor diagram of the series compensated system... 

We will notice subsequently that series and shunt compensation are complementary. What a shunt capacitor cannot do, a series capacitor does and vice versa. On a secondary transmission system, say up to 66 kV, a shunt compensation may always be necessary to improve the power factor, as the load would mainly be indtictive. A series compensation may become essential, to improve the stability of the system, to cope with load fluctuations, switching of non-linear loads and voltage fluctuations occurring on the other power system or the grid to which this system may be connected. 

In FIT systems too the concept is very similar. Now. besides the p.f., the stability of the system also defines the prerequisites for efficient power transfer over long distances. The tise of both shunt and series compensations may now be necessary to achieve the desired goal. 

Figure 24.11 An open circuited series compensated transmission line...

Figure 24.16(a) Series compensation by sectioning at the midpoint of the line... 

Since a shunt compensation will reduce Xq (° l/r), it will not provide the desired compensation. This can be achieved with the use of series compensation, C remaining the same. Then... 

This value can be reduced by decreasing the value of X , which is possible by providing series capacitors in the line. If X(2c is the series compensation, then the modified impedance... 

To overcome such situations within acceptable parameters during normal operation, it has been found that an ideal series compensation is achieved at around 40-70% of X, preferably in the range of 45-60% only. The level of compensation will depend upon the expected load fluctuations and the presence of harmonic disorders in the system. 

Series compensation would mean a low value of Zq and hence a higher system fault level. This needs be kept in mind while designing the system and selecting the switching devices or deciding on the protective scheme or its fault setting. 

Example 24.3 Application of series compensation on an HT distribution network... 

There is no leakage capacitance, Co, and hence no Ferranti effect on such low voltages. We will use series compensation to reduce the line voltage drop and improve the regulation and hence the stability of the network as well as its load transfer capability,... 

In Example 23.2 the system was not capable of transmitting its full capacity. Let us consider that with the use of series compensation it can be fully loaded up to... 

To study the impact of series compensation we consider the full-rated current of the transformer and the line for optimum utilization of the entire system. 

Central Board of Irrigation and Power, India, Workshop on. Series Compensation in Power. Systems, Nov. (1986). 

Application of series capacitors and analysis of an uncompensated transmission line and the capability of power transfer and system regulation with and without series compensation are also presented. 

The missile is to have a series compensator (design one) of the form... 

The performance of a transmission system can be improved by reactive compensation of a series or parallel type. Series compensation consists of banks of capacitors placed in series with each phase conductor of the line and is used to reduce the series impedance of the Kne, which is the principal cause of voltage drop. Shunt compensation consists of inductors placed from each line to neutral and is used to reduce the shunt susceptance of the line. 

The calculation model was represented as the direct current scheme because the GIC are considered to be the quasi Direct Currents (DC) since their frequency is much lower than the nominal operational frequency (50 or 60 Hz). In the same manner, transmission lines with installed series compensators were neglected, since they perform as barrier to GIC. 

FIGURE 10 Sketch of a 9000-MVA triggered vacuum gap used to protect the series-compensating capacitors on the 500-kV. Pacific intertie ac transmission line. Two communicating gaps in series with an interdigital paddle-wheel geometry were used to disperse the vacuum arc and avoid anode-spot formation. 

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